1. Field of the Invention
The present invention relates generally to the field of grid-connected inverter systems, and more particularly to a method and system for detecting the negative sequence for three phase grid-connected inverter systems.
2. Background of the Invention.
For grid-connected inverter systems, negative sequence detection is important for protection against grid fault, such as single-phase open and excessive unbalanced load, and to ensure smooth operation of the grid and prevent damage to the inverter or other equipment. In grid-connected inverter systems, if there is an imbalance of the load on one phase, it can cause damage to either the inverter or the other equipment. Therefore, it is important to detect such an imbalance, so that the inverter can disconnect and stop delivering power to the system. In order to detect a grid-connected inverter system imbalance, a technique that is typically employed is referred to as using the negative sequence to detect the imbalance. Either impedance unbalance or voltage unbalance causes unbalanced output phase current for grid-connected inverters. The negative sequence of three phase systems is often used as an indication of the system unbalance.
There are many ways to detect the negative sequence. One of the classical ways, as described in FIG. 1, requires a complicated calculation. The negative sequence of phase current can be expressed in terms of individual phase current (Ia, Ib, and Ic) and phase angle, theta, according to the negative sequence calculation shown in FIG. 1. The classical method illustrated in FIG. 1 also utilizes very special low pass filters to enable detection of the negative sequence. The low pass filters illustrated in FIG. 1 are necessary to increase the signal-noise ratio. However, these low pass filters limit the system performance because, for example, if the low pass filter is limited to 60 hz, then system performance cannot be detected past the cut-off frequency, and the system performance is limited by the low pass filter.
Referring to FIG. 1, the classical way of obtaining the negative sequence involves, for example, first multiplying the phase A current (Ia) times the cosine of the phase angle, theta; adding the difference between the phase C current (Ic) and phase B current (Ib) times the sine of the phase angle, theta, times one divided by the square root of three. This classical way also involves making the same calculation except with a negative sign. The result of each calculation is input through its own low pass filter, and the outputs are the real portion of the current (Ireal) and the imaginary portion of the current (Iimg). The real and imaginary portions of the current are then used for a root mean square calculation, in which negative current (Ineg) equals the square root of the sum of Ireal squared and Iimg squared. That is the classic calculation.
FIG. 2 illustrates an example of a waveform with a 25% imbalance, which means that the negative sequence is 25% of the positive sequence. The value of the Ireal is the magnitude of the alternating current (ac) of the wave, and the value of the Iimg is the direct current (dc) part represented by the solid horizontal line at 0.25 on the vertical axis. That is the dc offset, which is the portion of negative sequence. Assume, for example, that the three phases, Ia, Ib and Ic, are in perfect balance. In that case, after the calculation, the Ireal will be only a pure sine wave with no offset, which means that the center point of the sine wave will be at zero on the vertical axis. In FIG. 2, the vertical axis is the amplitude, which is 1.0, and the negative sequence is 0.25 or 25% of the positive sequence. In other words, the magnitude of the ac waveform is 1.0, and the curve is moved 0.25 upward on the plot. That is the offset that is caused by the negative sequence.
For example, if the input is 60 hz, Ia, Ib, and Ic, after the calculation shown in FIG. 1, Ireal will be 120 hz, and the Iimg part is dc, which is a constant. The severity of the dc offset is a measure of the severity of negative sequence, which is the classical way to measure the magnitude of the imbalance. It must be remembered that to measure the imbalance, it is necessary to relate the negative sequence to the positive sequence to determine the proportion. However, it is also necessary to detect the ac peak or the magnitude. In order to do that, the low pass filters are needed. The ac curve or waveform is the output of the first part of the calculation, and the low pass filter is used to smooth out the frequency so that the Ireal is filtered to a dc value. Thus, in the example, the Ireal is 1.0, and the Iimg is 0.25, which is a measure of the magnitude of the imbalance.
FIG. 2 shows that for a waveform for a 60 hz system before the low pass filters, the magnitude of negative sequence is 25% of the magnitude of positive sequence of the three phases Ia, Ib, and Ic of the alternating current. The frequency of the waveform is 120 hz, the amplitude of the waveform is the magnitude of positive sequence, and the direct current (dc)-offset of the waveform is the magnitude of negative sequence. The ratio of the amplitude versus the dc-offset of the waveform in FIG. 2 indicates the level of difficulty of designing the low pass filters. The higher the ratio, the more decay of the 120 hz component is needed to achieve good separation between signal (negative sequence, dc-offset) and noise (positive sequence, 120 hz). Also, the cut-off frequency of the low-pass filters, typically less than 60 hz for this case, cannot be too high. This causes a long time delay to detect the negative sequence in the classical method.
Because the output of the calculation block is 120 hz for a 60 hz system, the low pass filter typically requires a cut-off frequency that is lower than 60 hz, which causes a time delay in detecting the negative sequence. If the three current phases, Ia, Ib, and Ic, are changing rapidly, the changes will not show up on the Ireal, because of the low pass filter. The nature and purpose of the low pass filter is to smooth out the changes, so it tries to maintain the value to dc. If the inputs try to change, the purpose of the low pass filter is to smooth out those changes. Therefore, if Ia, Ib, and Ic change rapidly, they can become imbalanced, but the imbalance cannot be detected. Thus, the low pass filters used for the classical method prevent detection of imbalance in real time.
Moreover, the calculation for the negative sequence used for the classical method is a complicated, complex calculation process that requires greater computing power. The computation for the negative sequence according to the classical method, as shown in FIG. 1, involves, for example, eight multiplications, five additions/subtractions, one square-root, and two low-pass-filters. The calculation power required for the multiplications and square-root may force the microprocessor of the controller to a higher grade central processing unit (cpu), and, therefore, increases the cost.
It is a feature and advantage of the present invention to provide a method and system for real-time detection of the negative sequence of three phase grid-connected inverter systems, which enables the inverter to achieve grid-fault protection functions, such as single phase open, and disconnect itself from the grid.
It is another feature and advantage of the present invention to provide a method and system to simplify the detection of the negative sequence, which not only reduces the complexity of the calculation, but also improves the performance.
It is an additional feature and advantage of the present invention to provide a method and system for the detection of negative sequence that is simplified and therefore can be implemented either by software or by hardware or both.
To achieve the stated and other features, advantages and objects, an embodiment of the present invention utilizes, for example, computer hardware and/or software to provide a method and system for detecting the negative sequence for three phase grid-connected inverter systems. In an aspect of the present invention, a direct current component is removed from a direct-axis current feedback for a three phase current of the inverter to yield a pure alternating current waveform signal by subtracting the direct current component from the direct-axis current feedback. In this aspect, an input of the direct-axis component of a synchronous frame for at least two phases of the inverter system is received, and an amplitude of a frequency representing a magnitude of the negative sequence is extracted from the direct-axis component of the synchronous frame. The output of this aspect is the input for a first low pass filter.
The first low pass filter eliminates high frequency noise, such as natural high frequency noise from the inverter current input hardware sensing circuit, from the alternating current waveform signal while passing the negative sequence signal. The output of the first low pass filter is input to an absolute value function (ABS block), which rectifies the signal to an absolute value by removing a negative sign from a value of the signal. The rectifier doubles the frequency of the signal and outputs the rectified signal to a second low pass filter. The second low pass filter, which can have a cut-off frequency, for example, at least double the cut-off frequency of a first low pass filter, flattens the rectified signal. The output of the second low pass filter is a waveform signal indicative of a magnitude of the negative sequence current. This output signal can be compared, for example, to a preset threshold value that is determined by a percentage of negative sequence current to be detected. If the detected percentage of negative sequence current exceeds the preset threshold value, for example, a cut-off signal can be generated to the inverter system.